During wood formation in higher plants, most glucose produced during carbohydrate metabolism is channeled to cellulose for secondary wall deposition. Djerbi et al., Cellulose 11: 301-12 (2004). Cellulose is a fibrous polymer consisting of linear chains of β-(1,4)-linked glucan molecules that crystallize to form microfibrils. Microfibrils impart the characteristic flexible strength of cellulose. Cellulose is synthesized in higher plants by large multimeric plasma membrane-bound complexes that form rosette structures at the ends of microfibrils. Somerville, Ann. Rev. Cell Dev. Biol. 22: 53-78 (2006).
Cellulose is valuable as pulp, as fiber, and as a starting point for the synthesis of commercially important polymers. Alterations to increase cellulose deposition are likely to have a repressive effect on lignin deposition. Hu et al., Nature Biotech. 17: 808-19 (1999). A reduction in lignin content in woody plants is desirable, as the industrial production of cellulose and chemical removal of lignin is costly and represents an enormous environmental challenge.
The biosynthesis pathway of cellulose is poorly understood at the molecular level. Genes from the cellulose synthase (CESA) family, and those encoding proteins for N-glycan synthesis and processing have been isolated in a large number of organisms. Nicol et al., EMBO J. 17: 5562-76 (1998).
An experimental system largely used to study wood formation, especially cellulose synthesis and deposition, consists in bending a wood tree so that tension wood (TW) is formed at the tension side of the stem. Andersson-Gunneras et al., Plant J. 45: 144-65 (2006). In Eucalyptus and Populus species, tension wood occurs typically on the upper side of leaning stems and allows the tree to reorient its axis. Tension wood is mainly characterized by xylem fibers with an extra thick gelatinous secondary layer (G layer) in their lumen. This G layer contains almost exclusively cellulose microfibrils with high crystallinity. Déejardin et al., Plant Biol. 6: 55-64 (2004). Because tension wood is enriched in cellulose but is deficient in lignin and hemicelluloses, it may be used to detect and analyze genes involved in the control of carbon flow into lignin, cellulose, and hemicellulose.
Andersson-Gunneras et al. (2006) identified genes highly expressed in TW that are involved in cell wall formation, such as genes involved in carbohydrate metabolism and cytoskeleton formation, as well as housekeeping genes and two genes with unknown function in Populus tremula (L.)×P. tremuloides (Michx). A C3HC4-type zinc-finger (RING finger) protein showed differential expression TW, but there are no data, in this study or in others, implicating its role in cellulose biosynthesis.
Zinc finger domains are relatively small protein motifs that bind one or more zinc atoms. They were first identified as a DNA-binding motif in transcription factor TFIIIA from Xenopus laevis, however they are now recognized to bind DNA, RNA, protein and/or lipid substrates. The RING-finger is a specialized type of Zn-finger of 40 to 60 residues that binds two atoms of zinc, and is probably involved in mediating protein-protein interactions. There are two different variants, the C3HC4-type and a C3H2C3-type. The C3HC4-type RING finger motif is found in a number of cellular and viral proteins, some of which have been shown to have ubiquitin E3 ligase activity both in vivo and in vitro. Laity et al., Curr. Opin. Struct. Biol. 11:39-46 (2001).
Considering the difficulties associated with traditional forest tree breeding, such as the slow progress due to their long generation periods and the difficulty of producing a plant with a desirable trait, developments in gene technology can reduce significantly the time required to produce a new variety of plant and allow closer targeting of traits considered desirable by the forest and pulp industries in specific trees species.